1. Field of the Invention
This invention relates generally to a multiple access communication (MAC) system, and in particular, to an apparatus and method of channel encoding and channel decoding using a multiuser symbol constellation confined to a hexagonal lattice with each user having a defined symbol set.
2. Description of Related Art
A multiple access communication (MAC) system is a communication system in which a large number of users share a common communication channel to transmit information to a common receiver. Examples of this are the uplink path in a mobile-to-cellular phone tower or in a ground-to-satellite communication system. While the goals of MAC system designs can vary, they are generally concerned with efficient use of channel bandwidth, ease of decoding multiple user transmissions, providing a mechanism for users to enter the system, and reallocating resources when users leave the system. These goals are often at odds with one another whereby optimizing one criterion generally impairs or diminishes the others. For example, optimum multiuser coding sets, to make the most efficient use of a channel, typically require an extremely expensive decoding procedure, which has complexity which grows exponentially with the number of users. These approaches typically only can work with a very small (i.e. less than 10) number of users.
It should be understood that the reference to the term ‘users’ is not limited to a number of individuals with wireless communication devices. For purposes of this invention, ‘users’ is broadly defined as any source of information bits. These bits can be in relation to reading/writing to a hard drive, processing the feeds from video cameras, data transfer on networks and inter-component communications between integrated circuits. Thus the reference to user is provided for convenience and not intended to be limiting.
Other approaches, such as disclosed in R. E. Learned, A. S. Willsky, and D. M. Boroson, “Low Complexity Optimal Joint Detection for Oversaturated Multiple Access Communications”, IEEE Transactions on Signal Processing, Vol. 45, No.1, pages 113–123, January, 1997, includes tree-structured cross-correlation symbols having decoding algorithms with complexity which grows polynomially with the number of users. Another well known coding set, quadrature amplitude modulation (QAM), can be decoded with complexity which grows only linearly with the number of users, but the spectral efficiency of this approach is lower than the previous approach. Effective MAC systems design then becomes the problem of balancing the tradeoff between these conflicting goals.
U.S. Pat. No. 5,790,606 issued Aug. 4, 1998 to Paul W. Dent and assigned to Ericsson Inc. of Research Triangle Park, N.C., discloses a method of communication between a plurality of spatially distributed mobile radio units and a radio network using the same radio frequency, comprising the steps of transmitting information symbols simultaneously from the mobile units on the same radio frequency, sampling a radio wave on the same radio frequency at different points in space using a plurality of spatially distributed antennas to produce spatial signal samples, jointly processing the spatial signal samples using an equalizer adapted to resolve intersymbol interference between the information symbols in order to reproduce the transmitted information symbols. The information symbols transmitted from the mobile units include at least one known symbol pattern within a given area which is different for each co-channel mobile unit. The equalizer calculates correlations with the orthogonal symbol patterns using an orthogonal transform which may be a Fast-Walsh transform, a Fast-Fourier transform, or a Walsh-Fourier transform. The disadvantage of this approach is that a plurality of spatially distributed antennas at the receiver is required.
A successive interference cancellation (SIC) detector known in the prior art first decodes a user with the most power, user N, and then this information is used to decode user N-1, etc. Such an SIC detector is described in a paper by Shimon Moshavi entitled “MULTI-USER DETECTION FOR DS-CDMA COMMUNICATIONS”, IEEE Communications Magazine, October 1996, P. 124–136.
The importance of a hexagonal lattice in a single user signal processing application is described in a paper by W. H. Mow entitled “FAST DECODING OF THE HEXAGONAL LATTICE WITH APPLICATIONS TO POWER EFFICIENT MULTI-LEVEL MODULATION SYSTEMS, IEEE, ICCS/ISITA, Singapore, 1992, P. 370–373. This decoding algorithm is constructed based on the fact that a hexagonal lattice can be partitioned into two rectangular sub lattices, which allows additive channel noise to be removed in an extremely efficient manner. Of course, the disadvantage of Mow's approach is that it is suitable only for a single-user system, such as a dedicated modem.
What is needed is a more efficient communications scheme for multiple access systems that allows multiple users to efficiently transmit and receive information while reducing the effects from noise.